A large number of chemicals are not mutagenic unless they are converted enzymatically to active metabolites. The host-mediated assay is a useful and versatile tool for determining whether chemical agents are being converted to mutagenic metabolites by the host animals. This assay has been well established and employed extensively with indicator organisms such as bacteria and yeast. In Neurospora crassa, the host-mediated system has also been developed. However, no extensive studies have been carried out. It is, therefore, necessary to extend the host-mediated assay system using Neurospora as an indicator organism to different animal species, different organs and different procarcinogens.